Device for determining the 3D coordinates of an object, in particular of a tooth

ABSTRACT

A scanner is used for scanning an object ( 3, 4, 5 ), in particular a tooth or a plurality of teeth or a dental cast. The scanner ( 1 ) comprises a projector ( 2 ) for projecting a pattern ( 7 ) onto the object ( 3, 4, 5 ) and a camera which comprises a recording optics and an image sensor ( 18 ). To improve such scanner, the recording optics comprises a first imaging optics ( 9 ) and a second imaging optics ( 10 ).

BACKGROUND OF THE INVENTION

This invention relates to a scanner for scanning an object, inparticular a tooth or a plurality of teeth or a dental cast, and adevice for determining the 3D coordinates of an object, in particular ofa tooth or a plurality of teeth or a dental cast.

Devices and methods for determining the 3D coordinates of an object areknown already. EP 299 490 B2 describes a method for making a dentalprosthesis, in which contour lines on the ground tooth and itssurroundings are produced, the lines are detected with an optoelectronicmeans, in particular a video camera, the detected values are entered ina computer, and the three-dimensional structure of the tooth and thedental prosthesis is calculated. By means of the structure thuscalculated, the dental prosthesis can be fabricated.

SUMMARY OF THE INVENTION

However, the detection of the 3D coordinates of objects located inregions which are hard to reach, i.e. in particular of teeth in the oralcavity of a patient, involves certain difficulties.

Proceeding therefrom, it is the object underlying the invention topropose an improved scanner for scanning an object, in particular atooth, and an improved device for determining the 3D coordinates of anobject, in particular of a tooth.

In accordance with the invention, this object is solved by a scannerwith the features herein. The scanner is used for scanning an object, inparticular one or more teeth, wherein one or more or all teeth can beprepared. The term of preparing on the one hand comprises the dentalpreparation, i.e., for instance grinding the stump of a tooth, and onthe other hand the preparation which is necessary for scanning a toothwith an optical measurement technique, for instance spraying white sprayonto the region to be scanned.

The scanner comprises a projector for projecting a pattern onto theobject and a camera which comprises a recording optics and an imagesensor, in particular a CCD sensor or a CMOS sensor. In accordance withthe invention, the recording optics comprises a first imaging optics anda second imaging optics. In this way, a stereoscopic pair is produced bythe scanner. The imaging optics can be located at a distance from eachother. The optical axes of the imaging optics can be arranged at anangle with respect to each other. Preferably, they are arranged at suchan angle with respect to each other that they are directed towards acorresponding region of the object or tooth. As a result, the region ofthe object or tooth thus is observed with two cameras.

The scanner of the invention in particular can constitute a miniaturizedscanner. It is particularly useful for scanning teeth in the mouth of apatient. It is, however, also particularly useful for other applicationsin which the objects to be scanned are hard to reach. In particular, thescanner of the invention can be used for intra-ear scanning, forendoscopic digitization and/or in hard-to-reach cavities and/or channelsof machines and/or apparatuses.

Advantageous developments are described herein.

Preferably, the recording optics comprises a beam splitter. The imagesfrom the imaging optics can be supplied to the beam splitter. The beamsplitter can project these images onto the image sensor. In doing so,the images from the imaging optics can each be projected onto differentregions of the image sensor.

It is possible to provide further imaging optics. In particular, twomore imaging optics can be provided, so that a total of four imagingoptics are present. Each image can be projected onto a separate quarterof the image sensor.

It is furthermore possible to provide further image sensors. Inparticular, one image sensor can be present for each imaging optics.

By taking a picture, a particular region of the object or tooth can becovered. To be able to cover the entire object or the entire tooth orseveral or all teeth and possibly also the surroundings thereof, aplurality of pictures can be taken sequentially. The individual shotscan be combined to a total object representation.

For this purpose, the scanner is moved around the object, in order tocover several or all regions of the object. Since the scanner makes eachindividual shot in its coordinate system, the movement of the scanneradvantageously should be detected, so as to be able to combine theindividual shots as detailed and accurate as possible. This procedure isreferred to as “registering” or “matching”.

In accordance with an advantageous development, the scanner includes oneor more sensors for detecting the location and orientation of thescanner. In this way, a tracking system can be formed for the scanner.

For detecting the location and orientation of the scanner, 6D datapreferably are supplied by the sensor(s). The 6D data consist of threetranslational data and three rotational data. As a result, both thetranslation and the rotation of the scanner are detected completely. Inthis way, a tracking system is formed for the scanner.

In accordance with another advantageous development, the scannerincludes one or more acceleration sensors. The scanner movement can bedetected by the acceleration sensors, and the location of the scannercan be determined thereby. It is possible to detect the location of thescanner by the acceleration sensors.

In accordance with another advantageous development, the scannerincludes one or more gyrometers. The rotations, i.e. the orientation, ofthe scanner can be detected by the gyrometers.

The 6D information corresponds to the six degrees of freedom, which mustbe defined to unambiguously define a body, namely the scanner, in spacein terms of its location (position) and orientation (rotation). The 6Dinformation of the scanner is defined by three translatory componentsand three rotary components.

The temporal integration of an acceleration sensor supplies a velocity.The further temporal integration thereof supplies a path component.Proceeding therefrom, three acceleration sensors can be used. Sensorswhich determine all three translatory components at the same time are,however, also available.

The same is true for gyrometers, which detect the rotary accelerations,whose temporal integration two times over supplies an angle of rotation.It is possible to use three gyrometers, in order to obtain the threespatial angles of rotation of the scanner. However, there are alsogyrometers which supply all three spatial angles of rotation.

Furthermore, there are sensors which supply all three translations andall three rotations.

Instead or in addition, markers for a tracking system can be provided onthe scanner. The markers for the tracking system can be one or moreactive markers. In particular, infrared markers can be used, which aretriggered time-sequentially. However, the markers for the trackingsystem can also be one or more passive markers. In particular, thepassive markers can be coded and/or non-coded, reflecting and/ornon-reflecting markers. The markers can have different patterns.

The markers present on the scanner initially are surveyed. Hence, theposition of the markers on the scanner is known. The markers then aretracked by a tracking camera. By means of the tracking system, thelocation and rotary position of the scanner can be covered in thedifferent shots. In this way, possibilities for combining the differentindividual shots of the scanner are improved or created.

In a device for determining the 3D coordinates of an object, inparticular of a tooth, the object underlying the invention is solved bythe features of claim 12. The device comprises an inventive scanner forscanning the object or tooth and an evaluation means, in particular acomputer, especially a PC including the associated software, fordetermining the 3D coordinates of the object from the pictures taken bythe scanner.

Preferably, the device in accordance with the invention comprises atracking system for determining the location and orientation of thescanner. The tracking system can be formed in that the scanner includesone or more sensors for detecting the location and orientation of thescanner. In particular, the tracking system can be formed in that thescanner includes acceleration sensors and/or gyrometers. It is possiblethat the data of the sensor(s) for detecting the location andorientation of the scanner and/or of the acceleration sensors and/or ofthe gyrometer(s), in particular the 6D data thereof, are calculated backonto the location and rotary position of the scanner by temporalintegration. Instead or in addition, however, another tracking systemcan also be used, in particular an infrared tracking system.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be explained in detail below withreference to the attached drawing, in which:

FIG. 1 shows a scanner for scanning a group of teeth in a schematicview,

FIG. 2 shows the scanner of FIG. 1, which additionally includesacceleration sensors, and

FIG. 3 shows the scanner of FIG. 1 with infrared markers for an infraredtracking system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a scanner 1 which includes a projector 2 for projecting apattern onto the teeth 3, 4, 5. The tooth 4 is ground, the adjacentteeth 3, 5 are not ground and form the surroundings of the ground tooth4. The projector 2 comprises a light source 6, a pattern transparency 7and a projection optics 8. The light source 6 can be a light bulb or anLED. Instead of the pattern transparency 7 a transmitted-light LCD canalso be used, which can be activated for forming a pattern. The patterncan, however, also be projected by means of a DMD or LCOS method.

In the scanner 1, a first imaging optics 9 and a second imaging optics10 furthermore are provided, which are spaced from each other and whoseoptical axes 11, 12 form an angle with respect to each other. Thedistances of the imaging optics 9, 10 and the directions of the opticalaxes 11, 12 are chosen such that they are directed towards a commonregion of the ground tooth 4.

The scanner 1 furthermore comprises a beam splitter 13. In the opticalpath from the imaging optics 9 to the beam splitter 13 a first mirror 14is provided, which reflects the light which comes from the first tooth 4and is projected by the first imaging optics 9 onto the first mirror 15of the beam splitter 13. Correspondingly, a second mirror 16 is providedin the optical path of the second imaging optics 10, which reflects thelight coming from the ground tooth 4 and projected by the second imagingoptics 10 to the second mirror surface 17 of the beam splitter 13. Fromthe mirror surfaces 15, 17 of the beam splitter 13, the light isreflected to a CCD sensor 18. The image from the first imaging optics 9is projected onto the left half of the CCD sensor 18, the image from thesecond imaging optics 10 is projected onto the right half of the CCDsensor 18. The images from the imaging optics 9, 10 are supplied to thebeam splitter 13, which projects the same onto different regions of theCCD sensor 18. In this way, a stereoscopic pair is produced on the CCDsensor 18, which can be evaluated by a software present on a PC. The CCDsensor 18 can constitute a divided CCD chip.

The scanner shown in FIG. 2 corresponds to the scanner shown in FIG. 1,so that the components explained above will not be described again. Thescanner as shown in FIG. 2 additionally includes sensors for detectingthe location and orientation of the scanner 3. These sensors are threeacceleration sensors 19, 20, 21, which each supply data in x-, y- andz-direction, from which 6D data can be derived. These acceleration datacan be integrated temporally, so that the position in space and therotary position of the scanner can be determined therefrom. Theacceleration sensors 19, 20 are disposed in the handle 27 of the scanner1.

FIG. 3 shows the scanner 1 of FIG. 1 or 2 with cantilevers 22, 23, 24,at whose ends infrared markers 27, 28, 29; 30, 31, 32; 33, in particularinfrared diodes, are provided, which supply data for an infraredtracking camera 25. The infrared markers 27-33 emit infrared beams,which are received by the infrared tracking camera. The tracking system26, which consists of the markers 27-33 and the camera 25, can determinetherefrom the location and orientation of the hand-guided scanner 1.

The method can be performed such that the infrared diodes 27-33 light upone after the other. This is time-sequentially detected by the trackingcamera 25 (which can be a 3-line camera), from which the distance can bedetermined. The interconnection of the infrared markers 27-33 isunambiguously defined by its calibration. As a result, location andorientation in space can be calculated. The more markers can beevaluated, the more precisely can location and orientation be determined(some markers for instance are concealed during the measurement).

By means of the invention, a meteorological method can be performed,which is based on the evaluation of stereoscopic pairs. In the method,an object is observed with two cameras. With suitable algorithms, thesame features can be found and correlated in both images. In calibratedsystems, i.e. when the location and orientation of both cameras areknown, a distance value and hence a 3D coordinate—in the coordinatesystem of the scanner—can be calculated for each object point, which canbe observed in both cameras. The recording optics is equipped with abeam splitter. In this way, the scanner can be miniaturized. The objectto be scanned, in particular a dental preparation or an object in ahard-to-reach region of a machine or apparatus, can be observed from twodirections. The same scene is projected from two directions onto onlyone image sensor or CCD chip. An extension to more than two directions,in particular to four directions, is possible. All images can beprojected onto one CCD chip. It is, however, also possible to use aplurality of image sensors.

To find the same features of the object in the different images, apattern is projected onto the object. By means of the projected pattern,the associated image regions can be found in both images.

The pattern to be projected can be a stochastic or ordered pattern. Itcan consist of lines or cross gratings. It can be a time-constant ortime-variable or time-sequential pattern. The pattern can be anygraphical pattern (dots, lines, grating, etc.). The pattern can be agrey-scale pattern or a color pattern. The pattern can be projected withtransmitted light, for instance as a chrome mask (transparency) or as anLED projection. The pattern can, however, also be projected byreflection, for instance as an LCOS or DLP projection.

One image recording each can produce a data recording in the form of a3D aggregate of points. However, the same can represent only part of theentire representation of the real object. For this reason, a pluralityof individual shots of the object can be made successively, and theseindividual shots can be combined to form one total objectrepresentation. To achieve this, the scanner can be moved around theobject, in order to cover all regions of the object. Instead or inaddition, the scanner can be tracked, e.g. by an externally mountedtracking system.

The present invention creates a method and a device for dynamicallycovering the surfaces of objects, in particular for the dynamicintraoral coverage of the surfaces of dental preparations. A pattern isprojected onto the surface, which is observed from two or moredirections for generating the digital 3D data and is included in acorresponding number of 2D images. There can be effected aphotogrammetric evaluation of the 2D images, and the time-sequentialindividual 3D images calculated therewith can be combined by means of atracking system by determining the location and orientation of thescanner in space.

The photogrammetric evaluation can be an evaluation of stereoscopicpairs, which represent the object to be surveyed from two differentdirections. It can, however, also be an evaluation of several imagesfrom several views, in particular an evaluation of four images from fourviews. The stereoscopic pair can be an individual image, which consistsof two image halves which were taken from two viewing directions. Fourindividual shots can be combined in four “image quarters”.

The tracking system can be an optical or an interferometric trackingsystem. It is, however, also possible to realize the tracking system bysensors for detecting the location and orientation of the scanner, whichare attached to the scanner, and/or by acceleration sensors and/orgyrometers, by means of which the velocity and position of the scannercan be calculated back via the temporal integration. The accelerationsensors can supply three translational data. The gyrometers can supplythree rotational data, i.e. data on the orientation of the scanner. Itis particularly advantageous to provide both acceleration sensors andgyrometers, so as to obtain 6D data.

By means of the invention it is possible to determine the 3D coordinatesof an object. This can be accomplished in that the 3D coordinates arecalculated by correlating identical features in several, in particulartwo, images. By this method, associated identical image points can befound in both images of a stereoscopic pair, which were taken fromdifferent directions. By calibrating the imaging optics and bytriangulation, the 3D coordinates of the object points thus can becalculated. To be able to provide an unambiguous allocation of the samefeatures in the images, a pattern is projected. The projected pattern isused for this process referred to as feature recognition. In thismethod, the projector is not part of the calibration; it is independentof the stereo camera system.

It is, however, also possible to perform other methods for determiningthe 3D coordinates of the object. In particular, methods of the“single-image measurement technique” can be performed, i.e. methods withwhich the 3D coordinates of the object can be calculated from a singleimage recording. To be able to perform this method and to avoid aplurality of sequential image recordings of the same region, the picturetaken must contain all data necessary for calculating the 3Dcoordinates. To ensure this, a pattern usually is projected onto theobject. To be able to project this pattern, different properties of thelight can be utilized. The pattern can, for instance, be a cross gratingwith different colors, as it is described in DE 102 12 364 A1. In thismethod, the projector must be calibrated together with the camera. Themethod can be performed such that with the image from an imaging opticsand with the projected pattern the 3D coordinates are calculated andthat these 3D coordinates are correlated and optimized with thecoordinates calculated from an image of the other imaging optics andfrom the projected pattern.

The invention claimed is:
 1. A scanner for scanning an object (3,4,5)with a projector (2) for projecting a pattern (7) onto the object(3,4,5) and with a camera which comprises a recording optics and animage sensor (18), wherein the recording optics comprises a firstimaging optics (9) and a second imaging optics (10) and a beam splitter(13) positioned to receive light respectively projected from the firstimaging optics (9) and second imaging optics (10) and then reflectingthe light to the image sensor (18) and additionally comprising first andsecond mirrors (14, 16) each positioned to respectively reflect thelight projected from the first and second imaging optics (9, 10) to thebeam splitter (13).
 2. The scanner according to claim 1, wherein thescanner (1) includes one or more sensors for detecting the location andorientation of the scanner (1).
 3. The scanner according to claim 2,wherein the one or more sensors supply 6D data.
 4. The scanner accordingto claim 2, comprising three said acceleration sensors (19, 20, 21) eacharranged for respectively supplying data in x-, y- and z-directions todetermine both spatial and rotary position of the scanner (1).
 5. Thescanner according to claim 4, additionally comprising a handle (27) onwhich the acceleration sensors (19, 20, 21) are mounted.
 6. The scanneraccording to claim 1, wherein the scanner (1) includes one or moreacceleration sensors (19, 20, 21).
 7. The scanner according to claim 1,wherein the scanner (1) includes one or more gyrometers.
 8. The scanneraccording to claim 1, wherein markers (27, 28, 29; 30, 31, 32; 33) for atracking system (26) are provided on the scanner (1).
 9. The scanneraccording to claim 8, wherein the markers are active markers.
 10. Thescanner according to claim 8, wherein the markers are passive markers.11. The scanner according to claim 8, additionally comprisingcantilevers (22, 23, 24) on which the markers (27, 28, 29; 30, 31, 32;33) are mounted.
 12. The scanner according to claim 11, additionallycomprising a handle (27) on which the cantilevers (22, 23, 24) aremounted.
 13. A device for determining the 3D coordinates of an object(3, 4, 5), in particular of a tooth or a plurality of teeth or a dentalcast, with a scanner (1) for scanning the object (3, 4, 5) andevaluation means for determining the 3D coordinates of the object (3, 4,5) from the images taken by the scanner (1), comprising a scanner (1)according to claim
 1. 14. The device according to claim 13, comprising atracking system (26) for determining the location and orientation of thescanner (1).
 15. The scanner according to claim 1, wherein the beamsplitter (13) additionally comprises first and second mirror surfaces(15,17) each positioned to respectively reflect light reflected from thefirst and second mirrors (14, 16) to the image sensor (18).
 16. Thescanner according to claim 1, wherein the projector (2) comprises alight source (6), pattern transparency (7) and projection optics (8)arranged in this order for projecting the pattern (7) onto the object(3, 4, 5).
 17. A scanner for scanning an object (3,4,5) with a projector(2) for projecting a pattern (7) onto the object (3,4,5) and with acamera which comprises a recording optics and an image sensor (18),wherein the recording optics comprises a first imaging optics (9) and asecond imaging optics (10) and a beam splitter (13) positioned toreceive light respectively projected from the first imaging optics (9)and second imaging optics (10) and then reflecting the light to theimage sensor (18), markers (27, 28, 29; 30, 31,32; 33) for a trackingsystem (26) are provided on the scanner (1), and additionally comprisingcantilevers (22, 23, 24) on which the markers (27, 28, 29; 30, 31,32;33) are mounted, and three said cantilevers (22, 23, 24) and seven saidmarkers (27, 28, 29; 30, 31,32, 33), with two cantilevers (22, 23) eachhaving three markers (27, 28, 29; 30, 31, 32) mounted thereon and onecantilever (24) having one sensor (33) mounted thereon.